Estimating discriminating doses of phosphine for adults of eight species of psocids of genera Liposcelis (Psocodea: Liposcelididae) and Lepinotus (Psocodea: Trogiidae)

Phosphine (PH3) resistance and tolerance contribute to the increased importance of psocids (Psocodea: Liposcelididae) as stored-product pests. This study estimated the discriminating doses (DDs) of phosphine for laboratory susceptible adults of Liposcelis bostrychophila (Badonnel), L. entomophila (Enderlein), L. decolor (Pearman), L. paeta Pearman, L. rufa Broadhead, L. obscura Broadhead, L. fusciceps Badonnel, and Lepinotus reticulatus Enderlein. Discriminating dose is defined as the upper limit of the 95% confidence interval (95% CI) of the concentration that kills 99% of the individuals in a test sample (LC99). Psocids were exposed to incremental phosphine concentrations of 0.0–300.0 ppm and 0.0–150.0 ppm over a 20-h and 72-h fumigation period, respectively, at 25 °C using a modified FAO Method No.16. The estimated DDs had a range of 65.6–697.3 ppm and 18.1–194.5 ppm over a 20-h and 72-h fumigation period, respectively. For the 20-h fumigation period, the eight psocid species had higher DDs than those established for all stored-product insect pests in the FAO Method No.16 publication. Liposcelis entomophila and L. decolor were the most tolerant psocid species over a 20-h and 72-h fumigation period, respectively. Moreover, L. entomophila required approximately 9.3 and 6.7x higher phosphine concentration compared to the most susceptible L. obscura and L. reticulatus over a 20-h and 72-h fumigation period, respectively, to achieve similar levels of LC50 mortality. The higher heterogeneity values in the standard 20-h fumigation period indicate the potential for increase in phosphine resistance in field populations subjected to phosphine fumigation. Discriminating doses estimated in this study can be used for detection of phosphine resistance and estimation of resistance frequencies in field-collected populations of the psocid species investigated.